Nuclear fuels spent in nuclear facilities such as nuclear power plants are typically transported to reprocessing plants and then reprocessed. However, such spent nuclear fuels today are generated in an amount exceeding the reprocessing capacity. Thus, it is necessary to store spent nuclear fuels for a long period. In this case, spent nuclear fuels are cooled to a radioactivity level that makes the fuels suitable for transportation, and then placed in a cask as a nuclear shielding container and transported. Even at this stage, the spent nuclear fuels still emit radiation such as neutrons. Neutrons have high energy, and generate γ-rays to cause serious harm to the human body. For this reason, it is necessary to develop a material that surely shields such neutrons.
Neutrons are known to be absorbed by boron. To make boron absorb neutrons, it is necessary to slow down the neutrons. Hydrogen is known to be most suitable as a substance for slowing down neutrons. Accordingly, a neutron shielding material composition must contain a large amount of boron and hydrogen atoms.
Further, since spent nuclear fuels or the like as a neutron source generate decay heat, the fuels are heated to a high temperature when sealed for transportation or storage. Although the highest temperature varies depending upon the types of spent nuclear fuels, it is said that the temperature of spent nuclear fuels for high burnup may reach about 200° C. in a cask. For this reason, a nuclear shielding material for use preferably endures under such high-temperature conditions for about 60 years as a reference storage period for spent nuclear fuels.
In this situation, use of a substance having a high hydrogen density, in particular, water as a shielding material has been proposed, and some of the proposals have been put into practice. However, water is difficult to be handled because it is a liquid, and is not suitable for a cask for transportation and storage, in particular. Moreover, it is difficult to suppress boiling in a cask in which the temperature reaches 100° C. or more, disadvantageously.
Conventionally, a resin composition has been used as a material for a neutron shielding material, and an epoxy resin has been used in one of such resin compositions. Generally, there is a reciprocal relationship between hydrogen content and heat resistance in a resin composition. A resin composition having a high hydrogen content tends to have low heat resistance, and a resin composition having high heat resistance tends to have a low hydrogen content. An epoxy resin exhibits excellent heat resistance and curability, but tends to contain only a small amount of hydrogen indispensable for slowing down neutrons. Therefore, an amine curing agent having a high hydrogen content has been used to compensate this drawback.
Japanese Patent Laid-Open No. 6-148388 discloses a neutron shielding material composition which employs a polyfunctional amine epoxy resin to have reduced viscosity and improved workability at ordinary temperature and exhibits excellent pot life. Japanese Patent Laid-Open No. 9-176496 discloses a neutron shielding material obtained by curing a composition made of an acrylic resin, epoxy resin, silicone resin or the like with a polyamine curing agent.
Since an amine compound has a relatively high hydrogen content, the effect of absorbing neutrons is improved. However, the carbon-nitrogen bond contained in an amine curing agent is easily decomposed by heat. Accordingly, it has been demanded to develop a novel composition having durability necessary for storing a spent nuclear fuel for high burnup, rather than a conventional neutron shielding material made of a resin cured with an amine curing agent.
An object of the present invention is to provide a neutron shielding material composition which exhibits thermal durability improved as compared with a conventional composition, and surely absorbs neutrons.